Eta Carinae: A Tale of Two Periastron Passages

Since 2002, the far-ultraviolet (FUV) flux (1150-1680 angstrom) of Eta Carinae, monitored by the Hubble Space Telescope/Space Telescope Imaging Spectrograph, has increased by an order of magnitude. This increase is attributed to partial dissipation of a line-of-sight (LOS) occulter that blocks the central core of the system. Across the 2020 February periastron passage, changes in the FUV emission show a stronger wavelength dependence than occurred across the 2003 July periastron passage. Across both periastron passages, most of the FUV spectrum dropped in flux then recovered a few months later. The 2020 periastron passage included enhancements of FUV flux in narrow spectral intervals near periastron followed by a transient absorption and recovery to pre-periastron flux levels. The drop in flux is due to increased absorption by singly ionized species as the secondary star plunges deep into the wind of the primary star, which blocks the companion's ionizing radiation. The enhanced FUV emission is caused by the companion's wind-blown cavity briefly opening a window to deeper layers of the primary star. This is the first time transient brightening has been seen in the FUV comparable to transients previously seen at longer wavelengths. Changes in resonance line-velocity profiles hint that the dissipating occulter is associated with material in LOS moving at -100 to -300 km s(-1), similar in velocity of structures previously associated with the 1890s lesser eruption.

Automated summary

The far-ultraviolet flux of Eta Carinae has increased by an order of magnitude since 2002, attributed to the partial dissipation of a line-of-sight occulter blocking the central core. The 2020 periastron passage showed stronger wavelength dependence in FUV emission, with enhancements near periastron followed by transient absorption. The drop in flux is due to increased absorption by once ionized species as the secondary star plunges into the wind of the primary star.